Hey Jens,
Please git pull the following branch:
git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git stable/for-jens-4.5
which has three fixes for the Xen block drivers. They are quite simple
fixes. I was not entirely sure which branch to base them on - but they apply
just fine to for-4.5/drivers, for-linus, and for-4.6/drivers.